Aquantia PHY's of the AQR107 family support the downshift feature.
Add support for it as standard PHY tunable so that it can be controlled
via ethtool.
The AQCS109 supports a proprietary 2-pair 1Gbps mode. If two such PHY's
are connected to each other with a 2-pair cable, they may not be able
to establish a link if both advertise modes > 1Gbps.

v2:
- add downshift event detection
- warn if downshift occurred
- read downshifted rate from vendor register
- enable downshift per default on all AQR107 family members
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>